Phosphate conversion coatings are well known for treating metal surfaces, particularly ferrous, zinc and aluminum metals and their alloys. When applied, these phosphate coatings form a phosphate film, primarily of zinc phosphate or iron phosphate, which provides corrosion resistance and enhances the adhesion of subsequently applied coatings. Complete metal coverage and ultimately better phosphate coating properties can be achieved with phosphate crystals characterized by a fine and densely packed crystal structure. On the other hand, large, loosely packed phosphate crystals result in inadequate coverage which yields defective phosphate coating layers.
To achieve the desired crystal structure, the metal is usually conditioned or activated prior to applying the phosphate coating. Activation is done by contacting the metal surface with a diluted aqueous dispersion of a so called rinse conditioner or activator. These rinse conditioners or activators are also commonly known in the art as activating agents, nucleating agents, crystal refiners or grain refiners. For the purposes of this disclosure, the terms rinse conditioner or activator will be used.
Rinse conditioners are aqueous dispersions which contain Jernstedt salts such as those described in U. S. Pat. Nos. 2,310,239 and 2,456,947. Jernstedt salts are colloidal salts of disodium phosphate and a multivalent metal, such as, titanium, zirconium, lead and tin. Titanium (IV) phosphate salts are typically preferred for economic and environmental reasons.
The rinse conditioner is typically applied to the metal surface by immersing the metal in a bath of the rinse conditioner or by spraying the rinse conditioner onto the metal surface. It is believed the rinse conditioner creates nucleating sites such that when the phosphate conversion coating is applied, fine, densely packed crystals form.
Preparation of the titanium phosphate salt complex can be achieved in several ways under various conditions. In general, a titanium compound is reacted with phosphoric acid in the presence of sodium hydroxide, a sodium phosphate salt or combination of sodium phosphate salts, at a temperature of between 65.degree. C. to 100.degree. C. The salt is usually recovered as a solid and is transferred to the application site in this form.
A disadvantage of having the titanium phosphate salt complex in solid form is that it must be put in aqueous medium at the application site. This is usually accomplished by making a concentrated aqueous slurry which is then diluted with additional water or is metered directly into an immersion tank which contains a dilute aqueous solution or dispersion of the salt and which is being used to condition metal surfaces. Unfortunately, the concentrated aqueous slurries based on the sodium phosphate salts are not particularly stable and have a tendency to settle out and to lose their activity and plug the pumps associated with the metering equipment. These slurries typically require daily preparation and constant agitation to insure the activity of the rinse conditioner and to prevent solid material from clogging metering pump lines. It would therefore be desirable to formulate a rinse conditioner of the Jernstedt salt type which would overcome the problems associated with the Jernstedt salts recovered in solid form as mentioned above.